Remove all partitions from sd card, so its completely unallocated (with minitool for ex).
Flash box to original 1.2.6 image, and dont upgrade.
Turn off box en remove usb stich with image. (so nothing attached)
Turn box back on and dont run settings or upgrade.
Insert sd card and go directly to settings.
Go to usb / storage and click on bottom , tells unsupported sd card and run it as internal.
Ignore speed errors, if you dont see the card unplug and insert again until you see it. (sometimes 3x ?)
For asking to transfer files say also yes ! ( so kodi can use it)
Wait until finished en reboot.
Now its part of the total memory !
Upgrade throught ota en then to desired firmware.
Do not run factory settings or sd card will be lost again.!